1. Field of the Invention
The present invention relates to surface acoustic wave devices, such as surface acoustic wave resonators and surface acoustic wave filters, in which a surface acoustic wave element is sealed with a sealing member. In particular, the present invention relates to improvements in materials of the sealing member and in the crystal structure of an electrode of the surface acoustic wave element.
2. Description of the Related Art
As is generally known, a surface acoustic wave element is an electronic component utilizing a surface acoustic wave, which is generated by a mechanical vibrational energy concentrated in the vicinity of a solid surface and which is transmitted. In general, the surface acoustic wave element includes a piezoelectric substrate and an electrode for applying and transferring a signal, such as an interdigital electrode or a grating electrode, provided on the piezoelectric substrate.
The electrode is generally formed of aluminum (Al), which has a low electrical resistance and a low specific gravity, or an alloy primarily containing Al.
The surface acoustic wave element is, in practice, sealed in a package to define a surface acoustic wave device and is delivered as a finished product.
In general, the surface acoustic wave element is provided in a ceramic package with a metallic cover fixed by seam welding, and is thus, hermetically sealed.
However, an expensive ceramic package is required for such a packaging structure and the surface acoustic wave device cannot be easily miniaturized.
To provide inexpensive miniaturized surface acoustic wave devices, Japanese Unexamined Patent Application Publication No. 5-90882 discloses a packaging structure in which the functional surface of the piezoelectric substrate of the surface acoustic wave element, having the electrode thereon, is sealed with a resin or glass. This structure is commonly used.
However, resins and glass do not ensure hermetic sealing. Moisture penetrating from the outside through the resin or glass often reacts with the electrode on the piezoelectric substrate to change the operating frequency of the surface acoustic wave element, or gases from the resins or the glass cause corrosion of the electrode. Thus, the characteristics of the surface acoustic wave element deteriorate.
In order to prevent the moisture from reacting with the electrode, it is suggested that tantalum (Ta) or other suitable materials be provided in the Al electrode. Also, Japanese Unexamined Patent Application Publication No. 5-22067 suggests that the electrode be coated with a SiO2 protective film.
Unfortunately, the above-described additives for improving the corrosion resistance and moisture resistance of the electrode increases the electrical resistance of the electrode, thereby degrading the characteristics of the surface acoustic wave element.
Also, the protective film increases the weight of the electrode, thereby degrading the characteristics of the surface acoustic wave element.
If the moisture resistance and corrosion resistance of the electrode is improved without degrading the characteristics of the surface acoustic wave element, by sealing the element with a resin or glass, a miniaturized surface acoustic wave device having excellent characteristics can be achieved at low cost.